Wednesday, July 29, 2015

It was an interesting and exciting start to the week in the northern Rockies, northern Plains, and southern Manitoba. Snow, a long-lived tornado-producing storm, and hurricane force wind gusts were the result of an unusually strong low pressure system that developed over the northern Rockies on Monday and then rapidly intensified as it moved northeast into Manitoba. This system was fed by very warm, humid air to the southeast of the low and cold, dry air to the northwest, and was associated with a strong closed upper level low.

On Monday there was snow in Wyoming, Montana, and Idaho. There were several inches of snow above 8,000 feet elevation, including snow at ski areas in Jackson Hole, WY and in Glacier National Park in northwestern Montana. Flurries were even reported in Missoula. Temperatures on Sunday in western Montana and northwestern Wyoming were in the mid 80s, but on Monday daytime high temperatures were only in the low to mid 50s.

Surface temperatures at 5:00 p.m. MDT on July 27.

Strong thunderstorms developed northeast of the low pressure system late Monday, and one particularly strong storm developed over southwestern Manitoba during the early evening. This thunderstorm put down a tornado southwest of Melita, Manitoba around 8:30 p.m. CDT.

Radar image showing strong thunderstorm over southwestern Manitoba at 9:41 p.m. July 27. A hook echo is evident on the south end of the storm.

Radar base velocity image for 9:56 p.m. CDT on July 27. The white oval marks where rotation is indicated. Green colors are movement toward the radar. Red colors are movement away from the radar.

The tornado moved north-northeast over the next two and a half to three hours. At 10:55 p.m. CDT the storm was observed near Virden, and it's possible it lasted for a time after that but was not observed because of darkness.

Map showing approximate locations and times of the observed tornado.

It's possible that the thunderstorm produced multiple tornadoes rather than one very long-track tornado, but there was no doubt this was a monster storm. At one time the storm was estimated to be one kilometer (.62 mile) wide. Fortunately the tornado missed populated areas and there were no known serious injuries. The tornado did damage farm buildings, trees, power lines, and actually ripped pieces of asphalt from a highway. Meteorologists from Environment Canada have been investigating the damage and preliminarily assigned it a high EF-2 rating on the Enhanced Fujita scale. The damage survey has been hampered by the fact that the tornado did not hit many structures and spent much of its time over open fields. Of course, for the residents of the area that's a good thing.

On Tuesday the strong low was moving through Manitoba, and the intensifying low generated strong winds across southern Manitoba and North Dakota. Winds gusted to 70 mph and more at several locations in North Dakota, with the highest wind gusts of 76 mph reported at Garrison in central North Dakota and Alkabo in the northwestern corner of the state. Wind gusts in excess of 55 mph were common in western and central parts of the state. In eastern North Dakota winds gusted from 40 to 50 mph with the highest gust 59 at Devil's Lake and McHenry.

Surface map for Tuesday, July 28, 4:00 p.m CDT

Large-scale strong low pressure systems such as this are rare during the summer. This type of system typically occurs in the late fall to early spring, and often is associated with snow and blizzard conditions in the winter.

Thursday, July 23, 2015

At the start of this week there was some unusual weather in southern California - heavy rain. A low pressure system off the coast that used to be Hurricane Dolores and the southwestern monsoon flow combined to produce showers and thunderstorms from western Arizona west through southern California.

Visible satellite image of the southwestern U.S. at 4:00 p.m. PDT July 19. Green arrows show direction of winds.

Dolores attained hurricane status on July 13 off the central Mexican coast, then moved north northeast parallel to Baja California. It was downgraded to a tropical storm on Friday, July 17. By Sunday, July 19 it had weakened even further and was a post-tropical cyclone. The circulation was well-defined, and the low pumped moisture from the Pacific into northern Mexico and southern California. This augmented the moist flow of air associated with the southwestern monsoon, and the result was record-breaking July rainfall for many locations in southern California. San Diego has received 1.71 inches of rain in July so far which breaks the old 150 year-old record of 1.29 inches set in 1865.

Some rainfall records in southern California. From NWS San Diego.

One of our CoCoRaHS observers in San Bernadino County received 3.75 inches of rain over two days (by far the highest amount reported so far that I've seen), with a total for the month of 3.83 inches. Seven observers have measured more than 3.00 inches this month, and another several dozen observers have measured an inch of more. Not bad for an area where the normal July rainfall is just a few hundredths of an inch. Downtown Los Angeles has received 0.38 inches of rain this month. The normal is 0.01 inch.

24-hour CoCoRaHS totals the morning of July 20.

Total rainfall for the period from ~7:00 a.m. PDT on July 18 to &:00 a.m. PDT on July 20 in southern California.

As you might expect, that amount of rain caused problems. Many roads were flooded and washed out. Power was knocked out in some communities. A bridge on Interstate 10 between Coachella, CA and the Arizona state line washed out on the eastbound side and was compromised on the westbound side by flash flooding. The I-10 eastbound lanes are closed indefinitely until the bridge is repaired. The westbound lanes were opened again on Tuesday (July 21). About 27,000 vehicles normal travel this section of I-10 every day.

Firefighters stabilize a pickup truck that drove into washed out I-10 bridge. The driver was rescued.Photo credit: CalFire Riverside

On Sunday (July 19) the baseball game between the San Diego Padres and the Colorado Rockies in San Diego was first in a rain delay (6th time in history) and then eventually called off. It was only the second time in Petco Park history (since 2004) that a game has been rained out. The last rain-out occurred in 2006.

The rain did little to relieve drought conditions. The rain, while significant, is just a very small drop in a very large bucket. With evaporation rates on the order of 0.20 to 0.25 inch per day what didn't run off will soon be back in the atmosphere. The water balance chart from CoCoRaHS station CA-RV-21 in Riverside, CA shows just how little dent the recent rain made in the water deficit since May 1.

Drought conditions over the western U.S. and particularly California remain unchanged. They are likely to remain unchanged until the wet season begins in the winter, and then only if precipitation occurs regularly and is well above normal.

Wednesday, July 8, 2015

If you have frequented your local National Weather Service office's web site for your local point forecast through the "point and click" map interface you are familiar with the "forecast-at-a-glance" across the top of the page accompanied by icons depicting the type of weather expected. Effective July 7 the NWS instituted changes to these icons to make them even more representative of the weather expected. Changes include new graphic images for a variety of expected weather conditions. the ability to depict forecast for six-hour intervals, and colored "hazard boxes" to highlight watches, warnings, and advisories in the forecast period.

The icons weren't changed just for the sake of changing them. The changes were made based on research conducted by the National Center for Atmospheric Research (NCAR) in Boulder, CO, and by comments collected from users last year. Researchers surveyed people from across the country and found that most had been frequent and long-time users of the map interface. Researchers tested the former icons and the newly designed icons and presentation format. They found that the colored boxes drew attention to watches and warnings, and users were more aware of hazardous weather threats with the addition of start and end times to the hazard information. If you would like to read more about this research and evaluation see "Improving effectiveness of weather risk communication on the NWS point-and-click web page".

This is the sample forecast presentation that the NWS has on its page explaining the change in format. Note that in periods where the probability of precipitation is increasing or decreasing that change is shown on the icon. A yellow hazard box highlights the period of a severe thunderstorm watch, and also includes the icon for the first period of the forecast. The severe thunderstorm watch is also highlighted in the 12-hour forecasts in the background. The icon for Saturday depicts the expected weather for both the early and later part of the day.

If there are multiple watches and or warnings, these can be displayed as well. In the example below the forecast for Muskogee, OK for Wednesday depicts a Flash Flood Watch (yellow) into the evening (7:00 p.m. in this case) and a Flash Flood Warning (red) until 1:30 p.m. This information can be seen by clicking the "i" button at the top or on the multiple hazards box.

In the example below, a Red Flag Warning (favorable conditions for wildfire) is in effect for Jordan Valley, OR.

The point and click forecast pages on the NWS web sites receive on average 2 million hits per day, and during major severe weather events that number is substantially higher. If only, say, five percent of those viewing the page have a better understanding of the forecast and weather risk, that's tens of thousands of people who are more aware.

The NWS has a web page that lists all the weather conditions/forecasts and their associated icons (more than three dozen of them). They will continue to accept feedback about the new forecast-at-a-glance presentation and icons, and an email address to do so is provided on the page.

Tuesday, June 30, 2015

No doubt you have seen lightning strike displayed by your local TV station and other weather outlets. Where does that data come from, and how can you access it?

The lightning data you see shown along with radar displays likely comes from the National Lightning Detection Network (NLDN). The NLDN began operation as a regional network run by the State University of New York at Albany in 1983. The NLDN was eventually acquired by Global Atmospherics, Inc., and then in 2002 by Vaisala, Inc., a company that develops, manufactures and markets products and services for environmental and industrial measurement, especially meteorology and hydrology. The NLDN became national in coverage in 1989. It consists of over 100 remote, ground-based sensing stations located across the United States that instantaneously detect the electromagnetic signals given off when lightning strikes the earth's surface. These remote sensors send the raw data via a satellite-based communications network to the Network Control Center (NCC) in Tucson, Arizona. Within seconds of a lightning strike, the NCC's central analyzers process information on the location, time, polarity of the strike, and communicate this information to users across the country.

This lightning data is used by the utility industry, NASA, the National Weather Service, aviation, forestry, and many others. More information on the NLDN can be found here.

A map from Vaisala's "Lightning Explorer". Data on the map is 20 minutes delayed and updated every 20 minutes.

BlitzortungBlitzortung.org is sort of a CoCoRaHS for lightning. It is a world-wide lightning detection network for the location of electromagnetic discharges in the atmosphere (lightning discharges) based on the time of arrival (TOA) and time of group arrival (TOGA) method. It was developed by a few people in Germany several years ago, and since has expanded world-wide. This lightning detection network consists of volunteers with lightning detectors constructed from a kit developed by the Blitzortung group. The detectors transmit data to a central processing server over the Internet, which then processes the data to determine the location of lightning strikes. Other volunteers include programmers who develop and/or implement algorithms for the location or visualization of sferic positions (sferics are a type of radio signal produced by lightning), and people who assist to keep the system running. There are about 110 detection stations in the U.S. Lightning data is also available for Europe and eastern Australia.

The web site includes a world-wide live map of current lightning strikes, an archive of lightning data, and information on how to obtain a kit to build your own lightning detector. The construction of the detector requires some knowledge of and skills in electronics.

LightningMaps.orgLightningMaps.org is a community project with free lightning maps and applications. Real-time lightning data is available on a map-based interface utilizing the data from Blitzortung. New lightning strikes are depicted by yellow dots with red circle.The red circle disappears after 30 seconds, and the dots become darker as the time from the strike increases. There is an option to view the "thunder ring", a white circle that expands out from the strike at the speed of sound. There is also an option that allows you to turn on a layer showing the radar reflectivity.

Lightning strikes as displayed by lightningmps.org.

There are a number of apps available for your smart phone and tablet to alert you of nearby lightning strikes. One I like is free and uses the Blitzortung data feed.It's called "Blitzortung Lightning Monitor" and has some really nice features, including the ability to notify you of nearby lightning strikes. This is an Android version. I don't know if it is available for iOS.

The Blitzortung Lighting Monitor app. Click the image to enlarge and read the annotations.

Sunday, June 28, 2015

I certainly had an appreciation for lightning throughout my career as a meteorologist, but probably not enough as I should have. I've been outside when I should not have been, or stood on my porch watching thunderstorms as many of us probably have done. As I was gathering information for a web page on lightning and these blog posts, it was clear to me that I haven't been careful enough. The last two blog posts hopefully have made it clear - you don't mess with lightning - ever. Ben Franklin was one lucky guy, to say the least.

Take the threat of lightning seriously. Since most deaths and injuries occur outdoors, we'll look at these safety rules first.

“When Thunder Roars, Go Indoors!”

Credit: NOAA/NWS

This slogan was adopted by the National Weather Service several years ago, and while on the surface it might seem a little corny it gets right to the point .

If you are outdoors, find shelter in a nearby safe building or metal-topped vehicle with the windows closed. If you can hear thunder then you are at risk from lightning. The furthest distance from a lightning strike you can typically hear thunder is about five miles and seldom more than 10 miles, depending on atmospheric conditions. That "distant" rumble of thunder could only be several miles away, and just because you can’t hear thunder doesn’t necessarily mean you are safe. Lightning bolts are known to arc out tens of miles from the parent thunderstorm and may seemingly "come out of the blue". Stay inside at least 30 minutes after you last hear thunder.

Many injuries and fatalities from lighting were people who were headed to shelter but started to seek shelter too late. Pay attention to the weather and at the first sound of thunder or flash of lightning head to shelter. Fully enclosed buildings with plumbing and wiring provide the best protection. A hard-topped metal vehicle also provides protection. If you cannot find safe shelter there are some steps you can take to lessen your risk. However, it bears repeating that no place is safe outdoors in a thunderstorm.

Avoid open fields, the top of a hill or a ridge top.

Stay away from tall, isolated trees or other tall objects. If you are in a forest stay near a lower stand of trees.

If you are in a group, spread out to avoid the current traveling between group members.

If you are camping in an open area, set up camp in a valley, ravine or other low area. However, be aware of flash flooding potential in low-lying areas. Remember, a tent offers NO protection from lightning.

Stay away from water and wet items such as ropes, as well as metal objects such as fences and poles. Water and metal do not attract lightning but they are excellent conductors of electricity. The current from a lightning flash easily travels long distances.

Crouching doesn't make you any safer outdoors. Run to a substantial building or hard topped vehicle. If you are too far to run to one of these options, you have no good alternative. You are NOT safe anywhere outdoors.

However, the National Outdoor Leadership School along with NOAA recommends the lightning position when getting to safety is impractical or not possible. This may help minimize injuries if you are struck. You can download the complete brochure on backcountry lightning risk management here.

Lightning Safety Indoors

Stay off corded phones. You can use cellular or cordless phones.

Don't touch electrical equipment such as computers, TVs, or cords. You can use remote controls safety.

Avoid plumbing. Do not wash your hands, take a shower or wash dishes.

Stay away from windows and doors that might have small leaks around the sides to let in lightning, and stay off porches.

Do not lie on concrete floors or lean against concrete walls.

Protect your pets. Dog houses are not safe shelters. Dogs that are chained to trees or on metal runners are particularly vulnerable to lightning strikes.

Protect your property. Lightning generates electric surges that can damage electronic equipment some distance from the actual strike. Typical surge protectors will not protect equipment from a lightning strike. The National Lightning Safety Institute has information on protecting your home and electronics from lightning. Do not unplug equipment during a thunderstorm as there is a risk you could be struck.

Friday, June 26, 2015

Being struck by lightning is a life-changing experience for most people, and not in a good way. If you are fortunate to not be killed (only about 10 percent of people struck by lightning are killed), the injuries you suffer with may be with you the rest of your life.

There are five ways you can be struck by lightning.

A direct strike occurs when the person, usually in an open area, becomes part of the main lightning discharge channel. A portion of the current moves along and over the skin, and a portion moves through the body.

A side flash occurs when the lightning strikes a taller object near the person (like a tree) and part of the current jumps from that object to the person.

A person may also be affected by a ground current. When lightning strikes a tall tree, for example, the charge travels down the object to the ground and then along the ground surface. Ground current can cover a large area and is the cause of most lightning casualties. The current enters the body at the point closest to the lightning strike (for example, your foot) and exits the body at a point farthest away from the strike (your other foot). The greater the distance between these two points the greater voltage difference. The voltage difference is what drives the electrical current through your body and causes injury or death. Ground current is often fatal to livestock because of their large size, i.e. there is a large voltage difference between their front legs and rear legs and current travels trough the entire body.

Turf damage caused by ground current from a lightning strike. Photo credit: AlGamaty on Reddit

Conduction of lightning through wires or other metal surfaces allows lightning to travel long distances. Fences, electrical lines, pipes, or other metal surfaces can provide a pathway for lightning. Most indoor lightning casualties are related to conduction. That is why it is important to stay off of a corded phone, and stay away from anything plugged into an electrical outlet, water faucets and showers, or windows and doors.

Streamers develop as the downward-moving leader approaches the ground. These are upward streamers, and usually only one of the upward streamers makes contact with the leader to provide the main channel for the return stroke. However, when the main channel discharges, so do all the other streamers in the area. If a person is part of one of these streamers, they could be killed or injured during the streamer discharge even though they are not part of the main discharge.

While some lightning strikes result in death, the majority do not. However, disabilities from a lightning strike can be severe and long-term.

Most people can survive a lightning strike because much of the current dissipates over the skin (what is known as flashover)instead of entering the body. The electrical current is taking the path of least resistance, and can travel easier along the skin than it can within the body. When you hear about people who have had their shoes or clothes blown off it is because the flashover causes rapid heating of any moisture in the shoes or under the clothes (e.g. from sweat). The water vapor (steam) rapidly expands producing enough force to tear shoes or clothing from a person's body. This tends to occur with side flashes.

When lightning strikes your home it may damage your computer, television, and other electronics. When lightning strikes a person the primary injuries are to the body’s “electronics” – the nervous system and the brain. The most readily apparent effect may be cardiac arrest. Serious burns seldom occur. Most burns are caused by other objects (rainwater, sweat, metal coins and necklaces, etc.) being heated by the current passing through them and causing the burn rather than being caused by the lightning itself. The 90 percent of victims who are not killed by lightning exhibit various degrees of short and long-term disability.

Damage to the nervous system and the brain may not be readily apparent. Symptoms may include fatigue, intense headaches, inability to concentrate, inability to process information, personality changes, and others. Some symptoms may not manifest themselves until sometime after the incident. Often conventional medical testing (imaging, lab tests, etc.) will not show any physical changes that can be attributed to the lightning strike. Neurocognitive or neuropsychological testing may be used to identify functional and cognitive deficiencies.

The behavioral and personality changes that may be experienced by lightning-strike survivors are often hard for family and friends to understand. Lightning Strike & Electrical Shock Survivors International, Inc. is a non-profit support group formed by a lightning strike survivor in 1989. Its mission is to provide support for survivors, spouses, and other interested parties as well as to provide education on the prevention of lightning and electrical injuries.

"The Body Electric" is an excellent article on the experiences of and injuries suffered by lightning strike survivors.

John Jensenius, Jr., a Lightning Safety Specialist for the National Weather Service performed an analysis of 261 fatalities from lightning in the U.S. from 2006 through 2013. While many of us associate golfing with most lightning fatalities, that is in fact not the case. He found that fishermen account for the majority of deaths (30, vs. 8 for golf). Men accounted for 81 percent of all fatalities. Jensensius broke down the data into a number of categories, including age, sex, general type of activity (work, leisure), and specific activities within those categories. He found the two-thirds of those killed were involved in leisure activities.

From "A Detailed Analysis of Lightning Deaths in the United States from 2006 through 2013" by John Jensenius, Jr.

Wednesday, June 24, 2015

This week is Lightning Safety Awareness week in many parts of the country. If your local National Weather Service office is participating then you may have seen their links to some lightning information on their web page. I'll be sharing some information about this topic in my posts over the next several days.

Lightning is one of the oldest recorded natural phenomena. Despite our long study of lightning (remember Ben Franklin?) it remains on the frontier of atmospheric science.

Lightning occurs throughout the country and in all seasons. The area of highest incidence extends from the central and southern Plains through the Midwest and in the Southeast. Florida is the lightning capital of the U.S. with an average of 27 to more than 33 flashes per square mile per year.

As Ben discovered, lightning is a sudden electrostatic discharge from a thunderstorm. These giant sparks can extend from the cloud to the ground or objects on the ground, between clouds, within the cloud, or even between the cloud and air.

In many respects lightning is similar to the static electricity spark you may see or feel during the winter when the air is very dry and you touch a metallic object. When you walk across a carpet, for example, electrons move from the atoms in the carpet to you. You are, in effect, negatively charged. When you touch a metallic object like a door knob, the electrons move from you to the knob. The zap you feel and may hear are the electrons moving from you to the door knob through an electric spark.

Similar processes occur in a thunderstorm. As the thunderstorm develops the updrafts and downdrafts within the storm result in collisions between the precipitation particles within the cloud. Near the top of the storm these are usually small ice crystals. The ice crystals become positively charged and are carried higher into the storm because they are lighter. As a result the top of the storm becomes positively charged, while the middle and lower layers become negatively charged. Small ice crystals and small hail occur in the middle of the storm, while in the lower layer raindrops and melting hail occur. The collisions between these particles cause some to lose electrons and become negatively charged. The negative charge in the middle and lower layers of the thunderstorm cloud induces a positive charge in the ground underneath the storm, and the positively charged anvil induces the ground under the anvil to become negatively charged.

How electrical charges are distributed in a thunderstorm. Credit: NOAA

In the early stages of thunderstorm development the air acts as an insulating layer between the cloud and its surroundings. As the electrical charges build up within the thunderstorm, the difference between, for example, the negatively charged middle portion of the cloud and the ground become large enough to overcome the insulating effects of the air, and a lightning discharge occurs. When this discharge occurs between the middle of the cloud and upper portion of the storm, “in-cloud” lightning occurs.

Cloud-to-ground lightning occurs when the discharge happens between the cloud and the ground.

Cloud-to-ground lightning.Credit: UK Met Office

Cloud-to-ground lightning strikes account for about 25 percent of the lightning flashes worldwide. They are some of the most spectacular and also the most dangerous because they hit the ground or objects on the ground. The lightning discharge lasts only a few microseconds, but the process of its formation is complex.

Credit: NOAA

A lightning strike begins when an ionized channel of air, called a step leader, develops from the thunderstorm to the ground. As the step leader zigzags toward the ground, the electrical field increases as the quantity of positive charge residing on the Earth's surface becomes even greater. The electric field is strongest on grounded objects whose tops are closest to the base of the storm such as trees and tall buildings (that’s why you stay away from tall objects during a thunderstorm). This charge begins to migrate upward through buildings, trees and people into the air. When this upward rising positive charge – an upward leader or streamer – meets with the leader in the air above the surface, a conductive path is completed. Electrons surge along this path creating the visible lightning bolt. The rapid flow of electrons heats the surrounding air causing it to explosively expand, sending out a shock wave we hear as thunder.

Lightning is the third highest cause of weather-related deaths after flooding and extreme heat, causing an average of 51 fatalities a year with hundreds more injured. The National Lightning Safety Institute estimates that costs and losses due to lightning in the U.S. could be as high as $8 - $10 billion per year.

In my next post we'll look at how people are struck by lightning, the effects of being struck, and some statistics on lightning fatalities that may surprise you.

Wednesday, June 17, 2015

Tropical Storm Bill came ashore along the Texas Gulf Coast yesterday and since then has made his way well inland.

Although Bill was downgraded to a tropical depression this morning, there were still tropical storm force winds being recorded in Oklahoma this evening. The big impact from Bill is not the winds but the copious amounts of rain he's laying down along his path.

Rainfall was heaviest yesterday and last night in Jackson and Wharton Counties between Port O'Connor and Houston, TX. The CoCoRaHS observer at TX-JK-5 (Ganado 1.5 W) recorded a whopping 11.77 inches of rain for the 24-hour period ending this morning, and a total of 15.06 inches from June 14-17. There were a number of 24-hour amounts in excess of 7.00 inches in this same area. Fortunately there was almost two weeks of dry weather before Bill waded ashore.

24-hour precipitation ending the morning of June 17 in Jackson County, TX

4-day CoCoRaHS precipitation totals for coastal Texas near T.S. Bill landfall as of June 17.

As of noon today T.D. Bill was located north of Dallas and moving NNE. This evening the center of circulation was just south of Ardmore, OK.

Surface map for 7:00 p.m. CDT June 17, 2015

That's another part of the country that really doesn't need any more heavy rain. Expected rainfall in eastern Oklahoma is from 3 to 7 inches, with locally higher amounts. By Thursday and Friday the moisture associated with Bill will be moving into Missouri and the mid-Mississippi Valley, with three to four inches of rain expected there. This will be on top of already soggy ground resulting from the showers and thunderstorms associated with the frontal boundary that has been oscillating north and south this week in a very moist air mass.

There is some good news in all of this. Showers and thunderstorms are likely to drop some healthy amounts of rain from Indiana east through Ohio, Kentucky and through the mid-Atlantic coast. This area has been rather dry the past six weeks, with precipitation from 75 percent to less than 50 percent of normal.

Quantitative Precipitation Forecast (QPF) for the period from 6:00 p.m. CDT June 17 to 6:00 p.m. CDT June 20, 2015

It appears that rain gauges throughout the central and eastern U.S., including the Upper Great Lakes and the Northeast, will be getting a good workout in the next three to seven days.

Quantitative Precipitation Forecast (QPF) for the period from 6:00 p.m. CDT June 17 to 6:00 p.m. CDT June 24, 2015

The previous wettest May was in 1957 with 4.24 inches of precipitation,
and the previous wettest month was October 2009 with 4.29 inches of
precipitation.That one number representing the average precipitation across the lower 48 states is calculated using climate division data in each state. An average for each climate division is determined from the precipitation observations mapped to a 5 kilometer grid.

It's interesting to note that the May 1957 precipitation departures were
also highest in the southern and central Plains, but also extended west
to California. In all three cases the percent of the U.S. that was
classified as very wet (in the top ten percent of the historical
distribution) was 43 to almost 45 percent.

Percent of mean precipitation for May 1957 which is now the second wettest May on record.

Percent of mean precipitation for October 2009, now the second wettest month on record.

The fact this was a record wet May is not surprising news for those living in the southern and central Plains. It was the heavy precipitation over a rather large area that contributed to the record total. It was the wettest month ever in Texas and Oklahoma, and Colorado recorded its wettest May on record. It was the second wettest May in Arkansas, Kansas, and Utah.

Wednesday, June 3, 2015

The first tropical storm of the season, Ana, developed off of the southeastern U.S. nearly four weeks before the official start of the season. As Ana weakened and moved away, a late spring storm dropped from one to feet of snow on Colorado, Wyoming, and western Nebraska, with lighter but significant amounts in South Dakota and North Dakota.

California continued to suffer through another month of extreme drought, and the eastern U.S. along and east of the Appalachians was dry with less than 50 percent of average precipitation for most of the area. The notable exception was coastal North Carolina which benefited from the rains from the close approach of T.S. Ana. It was very warm across Alaska and many locations experienced a "top five" warmest May. In the eastern
Interior and northern Southeast Alaska this was the warmest May of
record. A high temperature of 91°F recorded in Eagle, AK on May 23 was the earliest in the season for 90°F or greater to be reached in Alaska.

In much of the U.S. between California and the Appalachians precipitation was much above normal, well more than twice normal from the Gulf Coast northward through the central Rockies.

The heaviest precipitation, however, was concentrated in Texas and Oklahoma, particularly in northeastern Texas through central Oklahoma.

May precipitation was very heavy from the Gulf Coast northward into eastern Kansas with amounts raining from 10 to almost 30 inches.

It was hard not to know what was going on during the three weeks from May 5 to May 26 if you paid any attention to the news. Oklahoma experienced its wettest month ever since 1895 with average statewide precipitation of 14.40 inches, 9.58 inches above normal and almost four inches higher than the previous record of 10.75 inches in October 1941. In addition, many locations in Oklahoma had their wettest month on record. Oklahoma City had 19.48 inches of rain during the month, beating the precious record of 14.66 inches set in June 1989. Normal may precipitation for Oklahoma City is 4.65 inches.

CoCoRaHS observers were there to document the historic rainfall. Of 204 Oklahoma CoCoRaHS stations reporting in May, an astounding 59 of them had 20 or more inches of rain during the month. There were 92 stations reporting on at least 28 days (90 percent) during the month, and of these the largest amount was 26.58 inches at OK-PG-7 (Krebs 0.3 WNW), with the lowest 4.48 inches at OK-CM-2 (Keyes 10.5 ESE)

A large portion of Texas experienced heavy rain during May, but the heaviest rain was concentrated in north-central Texas. Of the 1775 CoCoRaHS stations with reports in Texas during May, 672 reported a foot or more of rain during the month. The highest CoCoRaHS amount was 27.32 inches at TX-GA-9, Pottsboro 7.1 WNW in Grayson County along the Oklahoma-Texas border. A U.S. Cooperative observer in Gainesville, TX, (west of Pottsboro in Cooke County) reported a total of 28.90 inches for the month.

The heavy rain brought a dramatic end to the drought that plagued Oklahoma and Texas since 2010. As of the May 26 U.S. Drought Monitor only 5.4 percent of the state of Texas was in Moderate Drought or greater. The last time that number was this low was on May 11, 2010 when it was 3.51 percent. Five years ago this month there was no drought in Oklahoma, but the drought ramped up there in October 2010. It peaked in the fall of 2011. The drought disappeared briefly except in the Oklahoma Panhandle and the western half of Texas by the spring of 2012, but then reestablished and intensified, persisting at various intensities until this spring.

The drought in Texas in September 2011 (left), when it was at its peak, and on May 26, 2015.

The drought in Oklahoma on September 2011 (left) and on May 26, 2015.

This particular form of drought relief had other consequences as well. Significant flooding continues to occur along many rivers in Texas, particularly the Trinity River and the Red River. A number of locations along these rivers are expected to remain at major flood stage into next week.